The use of a chemical reaction to produce a continuous wave chemically pumped lasing action is well-known. The basic concept of such a chemical laser is described, for example, in U.S. Pat. No. 3,688,215. As therein described, the continuous wave chemical laser includes a plenum in which gases are heated by combustion or other means to produce a primary reactant gas containing dissociated atoms of a reactant element such as fluorine mixed with diluent gases, such as helium or nitrogen. These gases are expanded through a multiplicity of small supersonic primary or oxidizer nozzles to a high velocity and low pressure. The gases exit the nozzles into an optical cavity. A secondary reactant gas, such as hydrogen or deuterium, is injected into the cavity through secondary or cavity fuel nozzles located between the exit ends of the primary nozzles and mixed with the free jets of the primary reactant gas emerging from the primary nozzles and containing atomic fluorine. The resulting reaction between the hydrogen (or deuterium) and fluorine produces vibrationally excited HF or DF molecules. These molecules are unstable at the low temperature and pressure condition in the cavity and return to a lower vibrational state by releasing photons. Mirrors spaced in the cavity along an axis transverse to the flow field amplify the lasing action from the released photons within the optical cavity formed by the mirrors. The lasing action is of the continuous wave type which is pumped by the high energy vibrationally excited molecules formed in the optical cavity.
The lasing action depends on producing vibrationally excited states in the HF or DF molecules. This in turn requires that the molecules be formed under conditions of low temperature and pressure. As the pressure and temperature increase, the number of vibrationally excited molecules decreases and more energy goes into translational movement of the molecules, defeating the lasing action. To obtain low temperature and pressure in the optical cavity, the reactant gas is expanded through the primary supersonic nozzles to a Mach number greater than unity. Laser devices of the type described in the above-identified patent required large amounts of diluent gases in the flow to prevent excessive temperature increases. If the diluent gas is removed from the conventional chemical laser flow, large temperature rises occur as a result of the heat released by the exothermic reactions in the optical laser cavity. This temperature rise not only reduces the lasing action, but drives the Mach number of the flow to unity, thereby making recovery of stagnation pressure of the gas flow downstream of the laser cavity impossible. This phenomena is known as thermal "choking." The large amount of diluent gas requires large storage tankage and reduces the efficiency of the laser in terms of output light energy per lb. of input gases.
The use of conventional small scale primary or oxidizer nozzles as described in the above-identified patent leads to viscous flow through the nozzles resulting in a loss of total pressure, a decrease in gas exit velocity, and an increase in the gas exit temperature, also leading to a degradation in the lasing action.
It has been shown that this problem of viscous flow in small primary nozzles may be alleviated or eliminated while still maintaining high Mach numbers in the emerging gases, by replacing such nozzles by a single large primary nozzle and introducing the secondary reactant into the expanding primary reactant flow field emerging from the primary nozzle through a plurality of wedge-shaped reactant injection tubes traversing the opening of the single large nozzle. The thermal "choking" effect, described above, has been minimized by allowing the reactant gas to continue to expand after it emerges from the nozzle. However, in these prior art nozzle arrangements, expansion of the primary flow field occurs through so-called two-dimensional nozzle(s) whereby such expansion is essentially two dimensional in character and occurs predominantly in directions parallel to a given plane, referred to herein as the flow field expansion plane, parallel to the flow path of the flow field, and mixing of the primary and secondary reactants is accomplished by injecting the secondary reactant into the flow field through rows of secondary nozzle openings oriented normal to this expansion plane in such a way that molecular diffusion of the secondary reactant to effect reactant mixing occurs in directions parallel to this plane. Expansion of the flow field thus occurs in the same directions as the molecular diffusion of the secondary reactant to effect reactant mixing. Mixing of the reactants is thereby retarded and occurs along a substantial length of the primary flow field path. The concentration of excited molecules within the optical cavity and hence also the efficiency of the lasing action are correspondingly reduced. Thus, even though the total number of molecules in the vibrationally excited state may be sufficient to maintain a lasing action, the concentration of such molecules within the optical cavity is reduced, with a net loss of efficiency in the lasing action.